The proposed research aims to evaluate the hypothesis that reward anticipation selectively recruits the nucleus accumbens (NAcc) of the ventral striatum by eliciting a positive activated (PA) affective state. Study 1 will examine whether a functional magnetic resonance imaging (FMRI) probe of NAcc response during anticipation of monetary rewards of different magnitudes reliably indexes individual differences in reward sensitivity, as measured by affective reactions to reward cues. Study 2 will examine whether increasing anticipated reward probability will also increase NAcc activation and PA. A reliable neurobehavioral probe of reward responsiveness may have applications in assessing the severity of some affective disorders (e.g., anhedonic unipolar depression), the likelihood of recovery, and efficacy of pharmacotherapeutic or psychotherapeutic interventions.